HARD WICKE'S S CIENCE - G SSIP. 



241 



QUARTZ, AS IT OCCURS IN THE LAKE DISTRICT: 

 ITS STRUCTURE AND ITS HISTORY. 



PART II. 

 By J. CLIFTON WARD, F.G.S., F.R.M.S., &c. 



UARTZ as a Micro- 

 scopic Study : — 



Having enumerated 

 the various forms and 

 conditions under which 

 quartz is found in the 

 Lake District rocks, I 

 wish to say something 

 about its internal 

 structure as revealed 

 by the microscope. 

 The method of study is this : — The fragments of 

 quartz, or the rocks containing quartz, are sliced very 

 thin and mounted, the slice being reduced to such a 

 degree of thinness as to allow of its ready examination 

 by transmitted light. 



At the outset let us find out what can be the 

 difference between a piece of opaque white quartz 

 and clear transparent rock crystal. The difference is 

 somewhat similar to that between a piece of opaque 

 ice and a piece of glassy transparent ice ; for if water 

 be frozen very quickly, a large number of air-bubbles 

 will be entrapped, while slow freezing enables the 

 air to escape. Or, again, take a handful of snow, — 

 it is white, but decidedly opaque. Though formed of 

 clear crystals of ice, air is so mixed up with them 

 that the rays of light, constantly suffering reflection 

 from the limiting surfaces of the two, are unable to 

 pass through the mass. Subject such a mass of snow 

 crystals, however, to powerful pressure, the air will 

 be squeezed out and the formerly opaque snow may 

 be converted into a block of transparent ice. 

 Reverse the experiment ; pound up your block of 

 clear ice, and once more you have an opaque white 

 powder, the minute icy particles being separated from 

 one another by air. The same may be done with a 

 transparent quartz crystal : grind it to powder, and 

 the powder is found to be opaque and white. 



Hence, may we not conclude that the difference 



between clear rock-crystal and opaque white vein- 

 No. 167. 



quartz is that, while in the case of the transparent 

 quartz there is nothing or but little to reflect the rays 

 of light or to prevent their passing uninterruptedly 

 through the mass, in the case of the opaque white 

 quartz there must be something included which acts 

 in the same manner as the air-bubbles in opaque or 

 cat's ice, as it is called ? The microscope reveals 

 what this something is. 



Take a piece of ordinary vein-quartz, and examine 

 a thin slice of the same ; every part of the field of 

 view is seen to be full of little cells of very various 

 form, some round, some long drawn out, some 

 branched, and some even having a regular and 

 what appears like a crystalline outline. Examine 

 them more closely, and each little cell is seen to con- 

 tain a bubble, sometimes large, and more or less 

 flattened by the sides of the cavity, and immovable ; 

 sometimes like the bubble in a spirit-level, moving 

 sluggishly from end to end of a tube-like cell ; and 

 sometimes very small, and moving about freely in the 

 cell, like a thing of life, perhaps visiting all parts of 

 the cell in turn, occasionally becoming hazy and in- 

 distinct, because, passing to a deeper part of the cell, 

 the bubble has gone slightly out of focus. If the 

 cell, too, be very small, the bubble, owing to its 

 extreme minuteness, may seem like a mere black 

 speck ; and in these cases the motion is exceedingly 

 active, and reminds one still more of a living organism. 

 What can these things mean ? For a bubble to move 

 thus freely about, all analogy would lead us to sup- 

 pose that the cell must be full of liquid, except the 

 bubble-spot, otherwise this free motion would be 

 impossible. What, then, is the liquid ? and what does 

 the bubble contain ? Mr. Sorby has paid great atten- 

 tion to these interesting facts, and has clearly proved : 

 1st, that the liquid in most cases is water ; 2nd, 

 that the bubble is a vacuum, or empty spot, when 

 the movement in the cell is very free ; and 3rd, that 

 the water is frequently saline, and sometimes the 

 cells contain small cubical crystals of salt. Thus we 



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